1. Field of the Invention
This invention relates to tubing articles, and particularly relates to tubing articles suitable for use in irrigation systems. More particularly, it relates to tubing articles fabricated of fibrous plexifilamentary polyolefin material, and still more particularly, it relates to fluid permeable tubing articles fabricated of non-woven spun bonded fibrous plexifilamentary polyolefin material suitable for use in subsurface and drip or trickle type irrigation systems.
2. Background of the Invention
Tubing articles for use in subsurface and drip or trickle irrigation systems are known in the agriculture art.
"The Farm Quarterly", January--February, 1971, pp. 30-34 reviews the progress and promise of subsurface and drip or trickle irrigation systems. Early forms of tubing used in such systems were low cost plastic tubing having tiny holes or orifices punched therein at one or two foot intervals. Later systems for subsurface and trickle irrigation employ complex emitters. Both systems have the disadvantage of being susceptible to clogging of the holes or emitters from the soil itself or soil organisms or from particulate matter present in the supply water, e.g., silt, algae, etc. Early experiments with long fibred porous plastic tubing, having thousands of tiny holes ranging in size from about one micron to well over five microns are mentioned.
British Pat. No. 1,290,847 discloses the use of porous fibrous conduit fabricated from woven or non-woven structures of natural, regenerated natural or synthetic fibers.
The fluid permeable tubing articles of the present invention are not subject to clogging due to soil or soil organisms or from particulate matter carried in the supply water. Further, the tubing articles of the invention have high tensile strength to support the internal water pressure required in subsurface and drip irrigation systems, and high seal strength where edges of the tubing article are bonded, e.g., by heat sealing. Additionally, the tubing articles have low cost and long service life under the soil as the plexifilamentary polyolefin material is not bio-degradable.
According to the present invention, there is provided a flexible fluid permeable tubing article formed of an non-woven plexifilamentary polyolefin material, said material characterized by having a porosity .epsilon. between 0.5 and 0.7, by having a contact angle greater than 85.degree., and a Gurley-Hill porosity in the range 4 to 70 sec/100 cc. The tubing is formed from two, like, superimposed, elongated members the lateral edges of which terminate in flat sealed projections.
The plexifilamentary sheet material from which the tubing articles of the present invention are formed is described in U.S. Pat. No. 3,442,740, which is incorporated by reference herein. Suitable plexifilamentary polyolefin sheet material may be prepared according to the teachings of the aforementioned patent from strands prepared from solid high molecular weight, fiber forming polymers. Hydrophobic polymers are especially preferred for their water repellant properties. Hydrophobic polymers may generally be considered to be those having the property of not being wetted by water. Polymers which may be used are found in the class of addition polymers. Among addition polymers, the polyhydrocarbons are preferred, and particularly linear polyethylene is preferred. Other suitable addition polymers are; e.g., blends of linear polyethylene and minor amounts of branched polyethylene, polypropylene, polybutene, polyisobutylene, polybutadiene and blends thereof.
The sheet material made from the above polymers is in the form of an integral network of primarily ribbon-like, fibrous elements whose cross sections vary along the length of the elements. This structure may be readily identified by tearing the sheet thereby exposing some fibrous elements, removing one or more of these fibrous elements and examining consecutive, transverse cross sections under a magnification of 450X. The filmy, ribbon-like elements have an average thickness below 4 microns.
Within the sheet structure, the polymeric material is arranged in overlapping and intersecting layers. The overlapping is apparent in the sheet structure from the fact that substantial amounts of fibrous elements are aligned in directions transverse to each other. This can be readily determined as by tearing the sheet. It will be noted that fibrous elements are exposed running in the direction which bridge the tear. The torn portion is then torn in a 90.degree. transverse direction to the original tear. Fibrous elements are again noted which bridge the tear. These elements run in a transverse direction, thus establishing the overlapping and intersecting nature of the layers. If overlapping were not present, the fibrous elements would all appear to run generally in the same direction as they do in the as-spun plexifilamentary material described in Belgian Pat. 568,524.
One characteristic of the sheet material suitable for use in the tubing articles of this invention may be defined as the absence of parallel straight through holes or continuous tunnels in the sheet material. This characteristic is observable under magnification of 450X of the surface of the sheet. Under such observation it will be noted that there are discontinuous pockets or voids in the sheet material which randomly communicate by interconnecting tortuous paths with one another to provide the fluid permeable characteristic required for transmission of water at a controllable rate through the wall of a tubing article formed from the sheet material. Tortuosity in fibrous material is described in Flow Of Gases In Porous Media, Carman, Academic Press, New York (1956).
Suitable plexifilamentary polyolefin material in either sheet or strip form for use in fabricating the tubing articles of the invention is characterized by a porosity .epsilon. between 0.5 and 0.7, as defined by the ratio of free space or void volume to total volume of the material.
The plexifilamentary polyolefin sheet material from which the tubing articles of the invention are formed should not be readily wettable with water, e.g., the contact angle should be greater than 85.degree., as measured by the method described in Surface Chemistry; Theory and Applications, Bikerman, Academic Press, 2nd Ed., New York (1958).
The Gurley-Hill porosity of the plexifilamentary polyolefin material should be in the range of 4-70 sec/100 cc. and will preferably be in the range of 6-30 sec/100 cc. Gurley-Hill porosity, defined as the time required for 100 cc. of air to pass through 1 sq. in. of material at 1.25 psi, may be determined according to TAPPI Test Method T-460-M-49.
Suitable plexifilamentary polyolefin material for the tubing articles of this invention may have a basis weight between 1.3-3.5 oz./yd..sup.2 and a thickness between 4-12 mils.
The tubing articles of this invention may be formed by known methods of fabricating heat sealed tubing, e.g., the methods taught in U.S. Pat. No. 2,522,346 and U.S. Pat. No. 2,916,053.
In the first mentioned patent, a tube is formed by rolling up two plies of sheet material in a roll with convolutions of said plies separated by a sheet of non-heat sealable sheet material, the roll is sliced into narrower rolls, and heating the edges of the narrower rolls provides a heat seal between the longitudinal edges of the adjacent heat sealable sheets, while the non-heat sealable material between convolutions prevents a seal from being formed between convolutions of the heat sealable sheet material. The latter mentioned patent teaches a method of forming tubing by superposing layers of sheet material and drawing them over a plurality of heating elements disposed in spaced relationship to heat seal and slit the sheets into tubing of a desired diameter.